Which of the following metrics helps evaluate the performance of an email campaign?

Open Rate is a critical metric for evaluating the performance of an email campaign because it provides insight into how many recipients actually opened the email compared to how many were delivered. This metric is significant for understanding audience engagement and the effectiveness of subject lines, as a high open rate suggests that the email captured recipients' interest. Additionally, it serves as a precursor to other metrics, such as click-through rates, allowing marketers to gauge not only the initial engagement but also whether the email content encourages further interaction.

The other metrics listed, such as the number of followers, page views, and impressions, are more relevant to social media and web analytics rather than directly measuring email campaign effectiveness. The number of followers indicates overall audience size but does not reflect individual campaign performance. Page views track website traffic but do not provide insight into email engagement specifically. Impressions measure how many times content was displayed but do not reflect interaction or response to email campaigns. Thus, Open Rate is the most appropriate metric for evaluating email campaign performance.